In general, large-sized synthetic quartz glass substrates are prepared by furnishing a synthetic quartz glass block, slicing the glass block into plates by means of a slicing machine such as wire saw, lapping the quartz glass plates with a slurry containing abrasive grains, and subsequently polishing the plates until the desired size, thickness and flatness are reached.
If plate-shaped synthetic glass substrates as sliced by the slicing step are noticeably warped, the subsequent polishing step is required to meet that the substrate surface reaches a flatness within the predetermined range, to eliminate the warpage (or sori) of the substrate, and to maintain the overall thickness distribution within a certain range. Where the surface precision as sliced is constant, the necessary polishing allowance becomes larger as the warpage (or sori) is larger.
Then, the thickness of substrates into which the glass block is sliced is equal to the final target thickness plus the necessary allowance of the polishing step. If the magnitude of warpage (or sori) of substrates induced after slicing is not estimatable, then slicing must be done at an extra thickness including a certain margin. As a result, an extra polishing allowance is incurred for thickness adjustment during the subsequent polishing step, leading to losses of stock material and step time.
Highly flat substrates with minimal warpage are prepared, for example, by heat treating a substrate as sliced to deform it to mitigate warpage (or sori) and then polishing the substrate as disclosed in Patent Document 1, or by previously heat treating a block so that ingot-originating strains are concentrated in a peripheral portion thereof, removing the peripheral portion, and using the remaining block as stock material as disclosed in Patent Document 2.